1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member in which image exposure is conducted by use of laser light, in particular, an electrophotographic photosensitive member having excellent electric potential properties and excellent image quality when used in an electrophotographic apparatus which meets energy saving and a higher image resolution, and can also suppress nonuniformity and variation in sensitivity due to interference and further image defect caused by visualization of interference pattern.
2. Related Background Art
As a material for high-performance, high-durability and pollution-free electrophotographic photosensitive members used for copying machines and laser beam printers, amorphous silicon (hereinafter referred to as “a-Si”) deposited films compensated with hydrogen and/or a halogen element have hitherto been used. An electrophotographic photosensitive member using an a-Si deposited film has a structure which has a charge injection blocking layer having function to block charge injection from a conductive substrate, a photoconductive layer having photoconductivity, furthermore a surface layer having purposes of imparting capability for blocking charge injection, stable photosensitivity and the like, and other layers. Of these layers, the surface layer governs the electric and optical properties, properties relevant to the use environment and durability of the electrophotographic photosensitive member, and accordingly, surface layers containing various constituent elements and having various compositions have hitherto been proposed.
For example, Japanese Patent Application Laid-Open No. S57-115551 discloses an example of a photoconductive member provided with a non-photoconductive surface layer arranged on a photoconductive layer, wherein the photoconductive layer is constituted of an amorphous silicon material which is mainly composed of silicon and contains at least one of hydrogen atoms and halogen atoms, and the non-photoconductive surface layer is constituted of an amorphous material (a-SiC:H) which is mainly composed of silicon atoms and carbon atoms and also contains hydrogen atoms. Provision of the surface layer constituted of a-SiC:H makes it possible to improve the mechanical strength of the electrophotographic photosensitive member. However, when an a-SiC:H film is used as the surface layer, low-resistant substances such as moisture are adsorbed on the film in a high-humidity environment to tend to decrease the surface resistance and the charge retention ability, and consequently the electrostatic latent image pattern collapses to cause image defects such as image blurring and image deletion, so that sometimes a countermeasure against such resistance decrease of the surface layer is adopted in which the electrophotographic photosensitive member is heated. However, from the viewpoint of energy saving, it is demanded to unnecessitate such a heater. Accordingly, surface layers requiring no such a heater come to be proposed. For example, Japanese Patent Application Laid-Open No. S61-219961 (corresponding to U.S. Pat. No. 4,675,265) discloses an example of an electrophotographic photosensitive member in which a surface layer formed of an amorphous carbon (a-C:H) containing 10 to 40 atom % of hydrogen atoms is provided on a photoconductive layer formed of an amorphous silicon material. Because the surface energy of the a-C:H is small, the low-resistant substances are adsorbed in decreased amounts, so that the decrease of the surface resistance and the degradation of the charge retention ability can be suppressed in a high-humidity environment, resulting in that a heater to heat the electrophotographic photosensitive member tends to become unnecessary. However, an a-C:H film tends to absorb image exposure light, resulting in decrease of the sensitivity thereof. Additionally, while the electrophotographic photosensitive member is used repeatedly, an nonuniform abrasion thickness of the a-C:H film, if any, causes the sensitivity nonuniformity, which sometimes leads to the image density nonuniformity to degrade the image quality. As a surface material capable of overcoming such a drawback, Japanese Patent Application Laid-Open No. 2003-029437 discloses an example of an electrophotographic photosensitive member provided with a surface layer constituted mainly of magnesium fluoride. Magnesium fluoride has a low surface energy, and hence, the surface resistance and the charge retention ability are hardly degraded. Additionally, magnesium fluoride scarcely absorbs light, which makes it possible to suppress the sensitivity degradation.
In an electrophotographic photosensitive member having such a surface layer as described above, an intermediate layer is sometimes interposed between the surface layer and the photoconductive layer for the purpose of improving the degree of close contact, the electric potential properties, the image quality and the like.
For example, Japanese Patent Application Laid-Open No. 63-035026 discloses an electrophotographic photosensitive member having an a-Si intermediate layer containing, as constituent components, carbon atoms and hydrogen atoms and/or fluorine atoms. This intermediate layer makes it possible to reduce the cracking and exfoliation of the photoconductive layer. Additionally, Japanese Patent Application Laid-Open No. H2-203350 (corresponding to U.S. Pat. No. 5,262,263) discloses a technique in which the intermediate layer and the surface layer are formed of a-SiC:H and the surface electric potential is improved by appropriately regulating the carbon content in the interface between the photoconductive layer and the intermediate layer and the carbon content in the interface between the intermediate layer and the surface layer, and by reducing the dark decay.
The intermediate layer can be made to have an effect of improving the image quality. When an image is output by use of an electrophotographic photosensitive member in which such a surface layer as described above is deposited on the photoconductive layer, interference may be generated, when forming an electrostatic latent image by image exposure, to degrade the image quality; this problem can be overcome by providing the intermediate layer. For example, Japanese Patent Application Laid-Open No. H6-242624 (corresponding to U.S. Pat. No. 5,455,438) discloses an example of technique in which interference is prevented by avoiding formation of definite reflection planes, when forming the photoconductive layer and the surface layer by plasma CVD, by virtue of continuously varying the composition on going from the photoconductive layer to the surface layer. Additionally, Japanese Patent No. 2674302 (corresponding to U.S. Pat. No. 5,162,182) discloses an example of an electrophotographic photosensitive member having a charge transport layer, a charge generation layer and a surface layer laminated on a conductive substrate, wherein an interference-controlling layer is provided between the charge generation layer and the surface layer, the interference-controlling layer having a refractive index close to the geometric mean of the refractive indices of the charge generation layer and the surface layer and having a thickness so as to give an optical phase difference close to π/2 or 3π/2. Owing to these techniques, the manifestation of the interference can be suppressed, and accordingly image quality degradation can be prevented which is caused by manifest interference patters to be transcribed on the image.
Nowadays, in addition to improvement of image qualities such as image density nonuniformity and stability, the demand for higher image resolution has been increasing, and electrophotographic photosensitive members meeting the demand are desired.
For the purpose of enhancing the image resolution, it is effective to reduce the spot diameter of the exposure laser light. Examples of the methods for reducing the spot diameter of the exposure laser light possibly include the improvement of an optical system precision to irradiate the exposure laser light to the photoconductive layer, and the increase of the aperture ratio of the imaging lens. However, the spot diameter cannot be reduced beyond the diffraction limit determined by the wavelength of the exposure laser light and the aperture ratio of the imaging lens, and the requirements for the size increase of the lens and the mechanical precision improvement inevitably involve the increases of the apparatus size and the cost.
Accordingly, in these years, attention has been attracted to a technique in which the wavelength of the exposure laser light is made shorter to reduce the spot diameter so that the resolution of the electrostatic latent image may be enhanced. This is based on the fact that the lower limit of the spot diameter of the laser light is directly proportional to the wavelength of the laser light. In conventional electrophotographic apparatuses, laser light having oscillation wavelengths from 600 to 800 nm is generally used for image exposure, and further reduction of the wavelength can enhance the image resolution. In these years, development of semiconductor lasers having shorter oscillation wavelengths has rapidly progressed in such a way that semiconductor lasers having oscillation wavelengths in the vicinity of 400 nm have come into practical use.
For the purpose of enhancing the image resolution by means of the above described techniques, further improvement is required for the surface layer materials. For example, when the resolution is enhanced by reducing the spot diameter of the exposure laser light, there is a fear that even-such image deletion as nonconspicuous with a conventional spot diameter around 60 to 100 μm is sometimes manifested with an improved image resolution. Accordingly, for the purpose of improving the image resolution, it is necessary to form the surface layer by use of a material hardly causing image deletion.
Additionally, when an electrostatic latent image is formed by use of an exposure laser light having shorter oscillation wavelengths than the conventional oscillation wavelengths, the use of an electrophotographic photosensitive member having the surface layer formed of an a-SiC:H film or an a-C:H film makes larger the exposure laser light absorption in the surface layer to remarkably degrade the sensitivity of the electrophotographic photosensitive member. On the contrary, a magnesium fluoride film has a sufficiently small absorption to such a recently developed exposure laser light of a wavelength in the vicinity of 400 nm, and hence the sensitivity is hardly degraded. Magnesium fluoride is small in surface energy, and accordingly hardly causes image deletion in a high-humidity environment. Consequently, magnesium fluoride is promising as a surface layer material which can simultaneously meet both energy saving and higher image resolution.
Some problems to be overcome still remain in use of magnesium fluoride film for the surface layer. The present inventors have investigated the electrophotographic photosensitive member having a surface layer formed of magnesium fluoride, and have found that when magnesium fluoride is used for the surface layer on an amorphous silicon layer, desirable electric potential properties, particularly such as desirable charging ability, sensitivity and residual electric potential are sometimes hardly obtained. In addition, although metal fluorides such as magnesium fluoride hardly generate image deletion ascribable to the high-humidity environment, image defect accompanying image deletion sometimes tends to occur.
Moreover, when a magnesium fluoride film is used for the surface layer, the interference is manifested between the exposure laser light component which is reflected on the interface between the surface layer and the photoconductive layer and reaches the uppermost surface of the surface layer and the exposure laser light component which is reflected on the uppermost surface of the surface layer, and consequently sometimes the image quality is degraded. More specifically, a photoconductive layer composed mainly of amorphous silicon is often formed by the glow discharge method, in particular, the plasma CVD method using the electric power supply frequency of the RF band, VHF band or μW band because these methods are easy to control the operation conditions and capable of yielding excellent film properties. However, many of metal fluorides such as magnesium fluoride can hardly undergo film formation by the plasma CVD method, and accordingly, it is appropriate that a photoconductive layer is formed by means of a plasma CVD apparatus, and then a surface layer formed of a magnesium fluoride film is formed by use of a sputtering apparatus, a deposition apparatus or the like. The a-SiC:H film and the a-C:H film which have hitherto been used for the surface layer can be relatively easily formed by the CVD method, and the composition proportions of the elements constituting the layers can be continuously varied on going from the photoconductive layer to the surface layer to avoid formation of a definite reflection plane and to thereby prevent the interference; however, when a magnesium fluoride film is formed by sputtering or the like after an amorphous silicon film has been formed by the plasma CVD method, a reflection plane tends to be formed between the photoconductive layer and the surface layer. Consequently, interference tends to degrade the image quality when the exposure laser light tends to be reflected between the photoconductive layer and the surface layer because of the small roughness of the photoconductive layer surface and the like reasons. In order to overcome this drawback, an intermediate layer to suppress interference may be provided between the photoconductive layer and the magnesium fluoride film; however, in this case, it is necessary to appropriately select a material which can simultaneously ensure both the excellent electric potential properties and the suppression of the image quality degradation caused by interference.